1. Field of the Invention
The present invention relates to a semiconductor device. More particularly, it relates to a semiconductor device wherein source and drain regions having three regions formed by three different types of impurity doping steps are formed to prevent the occurrence of hot electrons which otherwise cause deterioration of the device performance.
2. Description of the Related Art
With the miniaturization of the semiconductor device the length of the gate electrode of a MIS FET has been shortened. The supply voltage is generally maintained at 5 V, and does not have a lowering tendency. This causes the problem that particularly in an n channel MIS transistor, the drain electric field is greater than in a conventional device, and a portion of the electrons accelerated by the increased electric field are injected into a gate insulating film: this is well known as a channel hot electron phenomenon. Further, a portion of the electrons generated by the impact ionization are injected to the gate insulating film and changes the characteristic of the MIS transistor; this is well known as an avalanche hot electron phenomenon.
In order to solve the problem of the hot electron phenomena, a double diffused drain (DDD) structure and a lightly doped drain (LDD) structure have been proposed since an electric field is not concentrated in the DDD structure, and thus the hot electron phenomenon is reduced. However, in the DDD structure, the effective channel length is decreased and thus a punch through phenomenon is generated. Further, problems of deterioration of the mutual conductance (g.sub.m) or breakdown voltage arise.
On the other hand, although the LDD structure has an effect on the channel hot electron phenomenon, it has little effect on the avalanche hot electron phenomenon, in which electrons generated at a deeper portion of the substrate due to the high electric field strength are accelerated so that the electrons are moved to the gate electrode through the gate insulating film. In addition, in the LDD structure, deterioration of the mutual conductance also occurs.